Conventionally, a diesel particulate filter must be periodically regenerated in order to reduce back pressure on the engine and/or to prevent a runaway exothermic soot oxidation reaction in a soot load trapped in the filter. Reducing back pressure on the engine is generally associated with more efficient operation, and hence an incremental reduction in fuel consumption by the engine. A runaway exothermic oxidation reaction is generally undesirable since temperatures can become briefly so high that the filter substrate (e.g., zeolite) may become cracked or otherwise damaged to the point that the filter may be compromised. Active regeneration of a diesel particulate filter refers to a process by which the accumulated soot in the diesel particulate filter is oxidized by increasing the temperature at the filter in order to encourage soot oxidation. The active regeneration process is sometimes carried out with fuel injected into an aftertreatment system upstream from the diesel particulate filter, or by the use of electrical heaters or the like. By initiating the regeneration process at a relatively low soot load density, the oxidation reaction can be controlled, and a runaway exothermic reaction, and the damage risks associated with such a reaction, can be avoided. However, there is often a tradeoff between the additional fuel consumption necessary to perform active regeneration of the diesel particulate filter verses the additional fuel needed by the engine to overcome back pressure associated with a soot accumulation on the diesel particulate filter.
In order to comply with the regulation of particulates and NOx, some engine manufacturers have implemented a strategy called selective catalytic reduction (SCR) in an attempt to reduce the presence of NOx at the tailpipe. SCR is a process where a reductant, most commonly urea ((NH2)2CO), a water/urea solution or the like, is selectively injected into the exhaust gas stream of an engine and absorbed onto a downstream substrate. The injected urea solution decomposes into ammonia (NH3), which reacts with NO in the exhaust gas to form water (H2O) and diatomic nitrogen (N2). Engine manufacturers implementing the SCR process typically include the diesel oxidation catalyst upstream of the SCR substrate to assist in altering the composition of the exhaust gas stream before it passes to the SCR substrate. Such diesel oxidation catalysts typically include a porous substrate made from, coated with, or otherwise including a catalyzing material such as palladium, platinum, vanadium, and/or other precious metals. Such materials facilitate a conversion of NO to NO2, thereby increasing the ratio of NO2 to NO upstream of the SCR substrate. The elevated level of NO2 provided by the oxidation catalyst may assist in both improving NOx conversion over the SCR catalyst and oxidizing soot particles that collect in a particulate filter.
Furthermore, conventional aftertreatment systems often contend with the presence of ash. In a given engine system, a complete seal between piston rings and engine cylinder wall may not be present. This allows some amount of engine oil to enter the combustion chamber causing the oil to burn. An ash residue remains and accumulates within the piston, ring grooves, and cylinder walls of the engine. Often this ash includes metallic elements and inorganic compounds originating from the additives present in the lubricating engine oil. Metal oxide particles are formed as a result of the combustion initiated oxidation of the metallic elements and inorganic compounds. These particles may then travel through the engine system into the aftertreatment system where they can collect on, and obstruct, the diesel particulate filter. These “ash” particles, however, are not susceptible to the filter regeneration process, as they are non-combustible. Conventionally, this ash layer is considered troublesome in that the ash blocks the filter, resulting in increased back pressure to the engine thereby increasing fuel consumption and decreasing power.
Although the conventional aftertreatment system structure has seen success and become somewhat widespread in use, there have been efforts to locate the NOx reduction reaction at the diesel particulate filter by coating the same with a NOx reduction catalyst. For instance, published U.S. Patent Application 2010/0058746 teaches a diesel particulate filter coated with both a diesel oxidation catalyst and a NOx reduction catalyst. However, this reference teaches a necessity of frequent active regeneration of the diesel particulate filter.
Certain strategies in aftertreatment methods, such as disclosed in published U.S. Patent Application 2012/0247085, have employed higher soot loading to optimize NOx reduction throughout the engine system. These systems however may not feature a diesel oxidation catalyst coated onto the diesel particulate filter. The lack of catalyst may ultimately result in an increase in the soot load density at which a runaway exothermic reaction soot oxidation can occur. The higher soot loading density has been found to deliver a reduced driving force on the nitrogen dioxide through a larger soot load layer, making the nitrogen and soot interaction time more similar to a conventional diesel particulate filter. Nevertheless, the higher soot loading on the particulate filter is still associated with the risk of runaway exothermic soot oxidation reactions, which can irrevocably damage the filter. These and other shortcomings of the prior art are addressed by the disclosure.